1. Field of the Invention
The present invention relates to a superconducting device which can control the critical current and a manufacturing method thereof.
2. Prior Art
Superconducting materials are available for manufacturing a superconducting device which can control the critical current.
Such materials were limited for a long time to those cooled by high cost liquid helium (boiling point 4.2.degree. K.), which is difficult handle. For example, such materials include metals such as lead and niobium, metallic compounds such as NbN, Nb.sub.3 Ge and Chevrel compounds, and metallic oxide such as Ba--Pb--Bi--O (BPBO) and Ba--K--Bi--O (BKBO). On the other hand, some oxide superconductors, including copper oxides, have recently been discovered to have a high T.sub.c above 100.degree. K., and they become superconducting when using the relatively low cost liquid nitrogen (boiling point 77.3.degree. K.) as a coolant. Thus, superconducting devices operated at liquid nitrogen temperatures are now available, and the application of such superconducting devices is accordingly expanded.
A superconducting triode, which is one of the superconducting devices which can control the superconducting current, includes a narrow strip of a superconducting thin film made of, for example, a metallic superconducting material Nb, and a metallic electrode arranged substantially perpendicular to the narrow strip and making contact with the thin film on an entire surface of a crossing or overlapping region of the thin film. The superconducting triode controls the superconducting current by injecting quasiparticles into the superconducting thin film from the metallic electrode to form a nonequilibrium superconducting region, the superconducting state in the overlapped region is weakened.
However, the prior art superconducting triode cannot control the width of the superconducting current path even though it can control the superconducting current by the injection of quasiparticles into the overlapped region of the thin film. Thus, if the width of the superconducting current path is much larger than the coherence length, a superconducting triode cannot operate as a weak-coupling bridge and thus the AC Josephson effect cannot be observed.
Further, if a high-T.sub.c oxide superconductor is used for a superconducting triode, the mechanism which causes the superconductivity has not yet been made clear, and the nonequilibrium superconducting state of such a superconductor has not been understood up to now because it depends strongly on the superconducting material.
Still further, a high-T.sub.c oxide superconductor suffer from drawbacks in that it partially dissolves due to the moisture in the air, and in that a solution, an acidic solvent or a basic solvent used in the manufacturing process may leave the superconductivity and damage the superconductor itself. That is, the surface of the thin film of the oxide superconductor is deteriorated during an improper manufacturing process, whereby the resistance value of the junction increases, thus deteriorating the superconducting properties and lowering the yield rate of the triode. For example, an oxide superconductor which includes copper cannot use a ziazo positive-type photoresist having an organic alkaline solution as a developer. Accordingly lift-off process which makes use of a ziazo positive-type photoresist cannot be adopted in the manufacturing processes of such a superconductor. Particularly, an oxide superconductor which includes yttrium is liable to suffer from oxygen damage whereby the superconducting properties become unstable by applying a metallic layer, and additionally the secular change of the superconducting properties is large.